Humping other dogs is not necessarily a sex thing; it can be part of normal play. But it is also one way dogs establish their place in the hierarchy--who ranks ahead of who. A teenage dog who is trying to figure out his place in the world is likely to try this behavior with other dogs. It basically is rude behavior on the part of the young dog, and most other dogs will put him in his place, either at first, or later when they decide it is too much to tolerate.
The problem here is that you’re looking at two male dogs, one a teenager, in a breed that has a tendency to same sex aggression, living in the same household. This kind of “sparring” to decide who is the boss is very likely to degenerate into out-and-out fighting, because neither dog will want to back down. That is major
trouble, not to be dealt with by the “casual” dog owner, who expects dogs to adapt to whatever lifestyle they are presented with--outdoor, indoor, multiple dog household, other pets present, perhaps limited attention (which is a problem for dobes), and so on...
Two male dobes in the same household are likely to need extra effort just to be able co-exist. They are not likely to ever be best buds. Bluntly, it just doesn’t sound to me like your family back home has enough specialized dog experience to know how to keep these two dogs together safely. That may involve a lot more effort to tailor their lifestyle to this particular dog’s needs than they had expected, and more than they are currently prepared to give.
And frankly, continuing on as they are currently doing is really not an option. Perhaps you can put together some good information for them to see.
This article might help them understand the humping issue: http://www.labadoption.org/info/file?file=16316.pdf
This forum has put together some links about same-sex aggression: https://www.dobermantalk.com/dt-educa...ggression.html
And then maybe encourage a family talk to discuss what adjustments they are willing to make so that family life runs smoothly for everyone (including the dogs themselves).